For the purpose of removing substances usually eliminated with urine and for the purpose of withdrawing fluid, various methods are currently employed for machine-aided blood cleaning or blood treatment in acute or chronic kidney failure. A method employing diffusive substance transport is typically employed in the case of hemodialysis (HD), whilst a method employing convective substance transport via a membrane takes place in the case of hemofiltration (HF). Hemodiafiltration (HDF) is a combination of these two methods.
An excessively high or rapid withdrawal of fluid during hemodialysis can give rise to a possibly rapid increase in blood volume, which often leads to an acute drop in blood pressure (hypotony) in the patient.
Hypotony represents one of the main complications in the treatment of blood. Various techniques have been proposed to deal with this problem. For instance, one technique employs blood pressure monitors which continuously monitor a change in blood pressure and regulate the ultrafiltration depending on the change in blood pressure. In another technique, blood volume monitors measure the relative blood volume during the dialysis treatment and perform a regulation of the ultrafiltration depending on the relative blood volume.
German Patent No. DE-C-197 46 377 describes a device for the measurement of blood pressure. This device is based on the detection of the propagation rate of the pulse waves being propagated via the arterial vessel system of the patient, these pulse waves being generated by the patient's heart contractions. The device permits a continuous, non-invasive measurement of blood pressure. One drawback of this device is that the pulse-wave running time is dependent on the hematocrit (HKT).
German Patent No. DE-A-40 24 434 describes a device for the regulation of ultrafiltration. With this device, the pressure in the extracorporeal circuit is measured in order to determine the relative blood volume. The measured pressure values are stored in chronological sequence and the change in the blood volume is determined from a change in the pressure value compared with the value at the start of the treatment. The venous return-flow or arterial suction-pressure sensor may be used as a pressure sensor. The reference describes that the drop in pressure on the arterial cannula is a function of the blood flow and the viscosity of the blood as well as a function both of the diameter and length of the cannula. It is also described or suggested that the relationship between the blood volume and the change in pressure is a substantially linear relationship.
One problem underlying the invention is to provide a method that permits the hematocrit and/or blood volume to be determined with a particularly high degree of accuracy, but with a relatively low technical outlay. Moreover, it is a problem of the invention to provide an apparatus for extracorporeal blood treatment with a device for determining the hematocrit and/or blood volume, which has a relatively simple construction, but a high degree of accuracy.